well, essential? I’d say from my personal opinion (in an obviously unreal sense because the body needs all systems) the endocrine system. Without the endocrine system you would be completely what? INSANE. Imbalances happen because either too much __________ is secreted or too little __________ is secreted. What if we had none? Those chemicals are there for a reason & have mad effects on the mind. We would all end up in mental institutions if we didn’t have it at all. Then again, of course, the human body is so complex. Every little crevice is there for a reason. Without all systems we would be an incomplete mess.